This invention relates to a process for the separation of C.sub.2+ or C.sub.3+ hydrocarbons from a feed gas stream containing light hydrocarbons in which the feed gas stream under a superatmospheric pressure is cooled, partially condensed and separated into a liquid and a gaseous fraction and in which the gaseous fraction is work expanded and the liquid fraction is fractionated by rectification into a product stream containing substantially all the C.sub.2+ or C.sub.3+ hydrocarbons, and a residual gas containing lower boiling components. By "light hydrocarbons" is meant a mixture of methane and at least C.sub.2-3 hydrocarbons. The feed gas may also contain components boiling lower than methane, e.g., nitrogen.
Such processes are frequently used for the separation of ethane and/or propane from natural gases or other hydrocarbon gases, for example, refinery tail gas. Furthermore, these processes are suitable for the separation of similar unsaturated hydrocarbons, for example, ethylene or propylene, assuming these components are contained in the gas stream to be fractionated, such as in a refinery tail gas. The reprocessing of refinery tail gas has recently become economically attractive since market prices for LPG (C.sub.3 /C.sub.4 hydrocarbon mixture) have risen, while on the othr hand vacuum residues and heavy oil are hard to sell. For this reason, the difficult to market heavy products are burned to cover the internal fuel needs of a refinery, while the easily marketable C.sub.3+ hydrocarbons are separated from the tail gas which, in particular, is evolved in large amounts during the processing of light crude oil components into gasoline.
Such a process which relates to the separation of C.sub.3+ hydrocarbons has already been described in German Patent Application No. P 34 08 760.5, filed Mar. 9, 1984 in Germany having a common assignee and corresponding substantially to U.S. application Ser. No. 709,742 filed Mar. 8, 1985 by Bauer et al, said U.S. application being incorporated by reference herein.
An essential feature of this German application is in the fact that the refrigeration obtained during work expansion of the gaseous fraction which remains after partial condensation is used not for the production of reflux liquid in the rectification column, but for cooling and partial condensation of the crude gas. Therefore, it is unnecessary to feed the light components of the gas stream into the rectification column. The elimination of the introduction of the light components present in the feedstock stream, particularly hydrogen, as well as C.sub.1 and if desired, the C.sub.2 hydrocarbons present in refinery gases; nitrogen, as well as C.sub.1 and, if desired, C.sub.2 hydrocarbons in natural gases into the rectification column makes it possible to perform the rectification at a higher temperature. The possibility of using a simple and economic cycle of external refrigeration for cooling of the overhead produced in the rectification stage indeed means a considerable improvement in carrying out the process, but yet additional improvement is desirable, nonetheless.